Lucky imaging

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Lucky image of M15 core
Lucky image of M15 core

Lucky imaging (also called lucky exposures) is one form of speckle imaging used for astronomical photography. Speckle imaging techniques use a high-speed camera with exposure times short enough (100 ms or less) so that the changes in the Earth's atmosphere during the exposure are minimal. With lucky imaging, those exposures least affected by the atmosphere (typically around 10%) are chosen and combined into a single image by shifting and adding the short exposures, yielding much higher resolution than would be possible with a single, longer exposure which includes all the frames.

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[edit] Explanation

Images taken with ground based telescopes are subject to the blurring effect of atmospheric turbulence (seen to the human eye as the stars twinkling). Many astronomical imaging programs require higher resolution than is possible without some correction of the images. Lucky imaging is one of several methods used to remove atmospheric blurring. Used at a 1% selection or less, lucky imaging can reach the diffraction limit of even 2.5 m aperture telescopes, a resolution improvement factor of at least five over standard imaging systems.

Zeta Bootis imaged with the Nordic Optical Telescope on 13 May 2000 using the lucky imaging method. (The Airy discs around the stars are diffraction from the 2.56m telescope aperture.)

Typical short-exposure image of a binary star, as seen using speckle imaging through the Earth's atmosphere.

[edit] History

Lucky imaging was first used in the middle 20th century, and became popular for imaging planets in the 1950s and 1960s (using cine cameras or image intensifiers). The first numerical calculation of the probability of obtaining lucky exposures was an article by David L. Fried in 1978.[1] In early applications of lucky imaging, it was generally assumed that the atmosphere "smeared-out" or "blurred" the astronomical images.[2] In this work, the FWHM of the blurring was estimated, and used to select exposures. Later studies[3][4] took advantage of the fact that the atmosphere does not "blur" astronomical images, but generally produces multiple sharp copies of the image (the point spread function has "speckles"). New methods were used which took advantage of this to produce much higher quality images than had been obtained assuming the image to be "smeared".

[edit] Lucky Imaging and Adaptive Optics Hybrid Systems

In 2007 astronomers at Caltech and the University of Cambridge announced the first results from a new hybrid Lucky Imaging and Adaptive Optics (AO) system. The new camera gave the first diffraction-limited resolutions on 5m-class telescopes in visible light. The research was performed on the 5m-diameter Palomar Hale telescope; the resolution achieved was twice that of the 2.5m diameter Hubble Space Telescope.

Lucky Imaging + AO image of the core of the M13 globular cluster. The best 10% of the frames taken were aligned and summed to make this final very-high-resolution (40 milli-arcsecond) image. Approximately 1-arcsecond diameter field.
Hubble Space Telescope ACS-camera image of the same field in a filter passing 660nm light. The stars in the Lucky Imaging + AO image are much better separated, although the Hubble image is longer-exposure and so shows some fainter stars.

When combined with an AO system Lucky Imaging selects the periods when the turbulence that the adaptive optics system must correct is reduced. In these periods, lasting a small fraction of a second, the correction given by the AO system is sufficient to give excellent resolution with visible light. The Lucky Imaging system sums the images taken during the excellent periods to produce a final image with much higher resolution than is possible with a conventional long-exposure AO camera.

This technique is applicable to getting very high resolution images of only relatively small astronomical objects, up to 10 arcseconds in diameter, as it is limited by the precision of the atmospheric turbulence correction. It also requires a relatively bright 14th-magnitude star in the field of view on which to guide. Being above the atmosphere, the much more expensive Hubble Space Telescope is not limited by these concerns and so is capable of much wider-field high-resolution imaging.

[edit] Popularity of technique

Both amateur and professional astronomers have begun to use this technique. Modern webcams and camcorders have the ability to capture rapid short exposures with sufficient sensitivity for astrophotography, and these devices are used with a telescope and the shift-and-add method from speckle imaging (also known as image stacking) to achieve previously unattainable resolution. If some of the images are discarded, then this type of video astronomy is called lucky imaging. Many methods exist for image selection, including the Strehl-selection method first suggested[5] by John E. Baldwin from the Cambridge group[6] and the image contrast selection used in the Selective Image Reconstruction method of Ron Dantowitz.[7] The recent development of EMCCDs has allowed the first high quality lucky imaging of faint objects.

[edit] Alternative methods

Other approaches that can yield resolving power exceeding the limits of atmospheric seeing include adaptive optics, interferometry, other forms of speckle imaging and space-based telescopes such as NASA's Hubble Space Telescope.

[edit] References

  1. ^ Fried, David L. (December 1978). "Probability of getting a lucky short-exposure image through turbulence". Optical Society of America 68: 1651-1658. 
  2. ^ Nieto and Thouvenot 1991, Recentring and selection of short-exposure images with photon-counting detectors. I - Reliability tests
  3. ^ Law et al 2006, Lucky Imaging: High Angular Resolution Imaging in the Visible from the Ground
  4. ^ Tubbs 2003, Lucky Exposures: Diffraction limited astronomical imaging through the atmosphere
  5. ^ Baldwin, John E.; Tubbs, Robert N.; Cox, Graham C.; Mackay, Craig D.; Wilson, Richard W.; Andersen, Michael I. (March 2001). "Diffraction-limited 800 nm imaging with the 2.56 m Nordic Optical Telescope". Astronomy and Astrophysics 368: L1-L4. doi:10.1051/0004-6361:20010118. 
  6. ^ Lucky Imaging Web Site Home
  7. ^ Dantowitz, Ronald F.; Teare, Scott W.; Kozubal, Marek J. (May 2000). "Ground-based High-Resolution Imaging of Mercury". The Astronomical Journal 119 (5): 2455-2457. doi:10.1086/301328. 
  • C. L. Stong 1956 interviewing scientist Robert B. Leighton for Amateur Scientist, "Concerning the Problem of Making Sharper Photographs of the Planets", Scientific American, Vol 194, June 1956, p. 157. Early example of exposure selection with mechanical tip-tilt correction (using cine film and exposure times of 2 seconds or more).
  • William A. Baum 1956, "Electronic Photography of Stars", Scientific American, Vol 194?, March 1956. Discusses the selection of short exposures at moments when the image through a telescope is sharpest (using image intensifier and short exposures).

[edit] External links